This application is a continuation in part of U.S. patent application Ser. No. 701,369, filed Feb. 13, 1985 and now abandoned, the subject matter of which is hereby incorporated by reference.
Devices used for producing heat by means of combustion of a fuel gas are in common use throughout the world. Typically, such devices are comprised of a gaseous fuel source connected to a control valve which is in turn connected to a burner wherein the combustion occurs. Many such devices also have ignition systems to initiate the combustion process.
In the past, most heaters had a small continuously burning pilot flame to provide a source of ignition. Since the cost of energy has risen in recent years, this standing pilot has gradually been eliminated, and replaced by the introduction of electronic ignition systems.
If any of the unburned fuel gas were to leak, a dangerous condition would result. For this reason, a flame detector is usually provided to shut off the flow of fuel gas if the combustion flame should cease. In small heating devices the safety system usually comprises a thermoelectric device such as a thermocouple placed in the pilot flame, which powers a safety electromagnet located within the control valve. When the electromagnet is powered, a normally closed valve with a closing spring biasing the valve closed is latched in the open position, allowing the flow of fuel gas. If the flame fails, the thermoelectric device cools, which reduces its electrical output. Eventually the output drops low enough that the electromagnet is released, allowing the closing spring to shut off the valve, terminating the supply of fuel gas.
Since the thermoelectric device typically has a large thermal mass, the cooling process takes a substantial amount of time. In fact 20 to 60 seconds may elapse before the valve closes. For small heaters, such a long delay can be tolerated. Larger heaters, such as those with capacities of 400,000 BTU/HR and greater, have a much larger gas flow rate, and a considerable amount of unburned gas may flow out of the burner if the valve stays open in the absence of a flame. For this reason, the larger heaters are required to have a flame failure response time not exceeding four seconds. Thus, thermoelectric devices are unsuitable. Also, since these large heaters usually have electric controls and igniters without standing pilots, the heat-up delay of typical thermoelectric sensors is also unsuitably long.
Many devices have been developed to provide the fast flame sensing function. Smith et al., U.S. Pat. Nos. 2,748,846, and Serber, 4,505,299, both describe a flame rectifier wherein the rectifying characteristics of the flame are exploited.
The use of optical sensing of the flame has been described for over 50 years. Ito et al., U.S. Pat. No. 3,765,820, describe the optical sensing of a luminous element in the flame for the purpose of controlling the fuel/air ratio and hence the combustion efficiency. Jones, U.S. Pat. No. 2,304,641, teaches that the direct light from the flame fluctuates whereas the emissive light from the hot burner surface is constant. The difference is exploited for flame detection. Ray, U.S. Pat. No. 2,408,954, teaches the use of a photomultiplier type light detector to directly sense the presence of the flame. Hapgood, U.S. Pat. No. 3,975,137, also uses the direct optical sensing of the flame, as does Crews, U.S. Pat. No. 2,388,124. Axmark and Satren, U.S. Pat. No. 4,370,557, look at the both the optical spectrum and flicker frequency characteristic of the flame. All of these devices commonly use the light issued by the flame directly or by the surrounding hot areas, as a signal to additional, independently powered electric or electronic circuits to sense the presence or absence of a flame. The flame rectifiers also require such external circuitry. Since the gas valve safety circuit is powered by the external circuit under the control of the flame sensor, it is possible that a failure of one or more elements in the external circuit could open the gas valve and cause a dangerous situation to occur.
The present invention is an extension of the principles taught in our previous copending application PCT/US No. 84/01038. In this invention, the photovoltaic control system described in the previous application is extended to include electrically controlled gas valves. An emissive element is placed in ,the flame. A photovoltaic device converts a portion of the resultant visible and infrared radiation to electric power which is used to power the safety electromagnet of the valves. The valves are adapted with safety electromagnets which are functionally identical to the safety electromagnets used in thermoelectric safety systems, but they are powered, not by thermoelectric devices, but instead by the photovoltaic device without the need for external amplifying circuits or external power supplies. All power for the safety circuit is derived from the flame only. Thus, the fast response time that is characteristic of the emissive element/photovoltaic system is obtained, and the system is inherently safe since no failure of any element of the safety circuit can lead to unintended opening of the gas valve.